Within the medical field, ultrasound systems are used for various imaging and treatment purposes. For example, there is an increasing appreciation of the diagnostic value of obtaining cross sectional images of coronary arteries by the method of intravascular ultrasound (IVUS). There are currently two general types of IVUS catheter systems. In a first type, subsets of an array of ultrasound transducers are sequentially excited in a manner to electronically steer an ultrasonic beam. This approach is sometimes referred to as the synthetic aperture focusing technique (SAFT). U.S. Pat. Nos. 4,917,097 to Proudian et al. and 5,186,177 to O'Donnell et al. describe use of this approach.
The second approach in the design of an IVUS catheter system is one in which the ultrasonic beam is redirected mechanically, rather than electrically. There are three subclasses of this mechanical approach. IVUS systems in the first subclass include either a rotating transducer or a rotating mirror in the distal end of the catheter that enters the vessel. A motor is coupled to the catheter at the proximal end that remains at the exterior of the bony. A drive shaft connects the proximal motor to the rotating distal transducer or mirror. U.S. Pat. Nos. 4,794,931 and 5,000,185 to Yock teach this technique. In the second subclass, rotation is confined to the distal end. U.S. Pat. Nos. 5,176,141 to Bom et al. and 5,240,003 to Lancee et al. teach incorporation of a micro motor at the distal end for rotating a transducer or a mirror. Alternatively, a fluid-driven turbine may be used to rotate the transducer or the mirror, as taught by U.S. Pat. No. 5,271,402 to Yeung et al., which is assigned to the assignee of the present invention. The third subclass is one in which the ultrasonic beam is generated at the proximal end of the catheter and is channeled to the distal end via a rotating waveguide, as taught by U.S. Pat. No. 5,284,148 to Dias et al., which is assigned to the assignee of the present invention.
The mechanical-rotating approach of directing an ultrasonic beam from a distal end of a catheter is more prevalently used than the approach of electronically aiming the beam. The mechanical approach can be implemented using a single transducer, while the electronic approach requires an array of transducers to be contained in the distal end that must pass through a vessel, such as a blood vessel. However, one concern in the use of an IVUS imaging system in which mechanical rotation is required is that the rotational velocity of the rotating structure will be nonuniform. A nonuniform rotational velocity will distort the image that is formed. One cause of nonuniformity with respect to the rotational velocity is the existence of mechanical friction and binding of the catheter as it spins in the tortuous path of the coronary arteries. Although the proximal end of the catheter is rotating at the desired velocity, any binding of the catheter along its length will lead to a distal rotational velocity that is different than the desired velocity. Assuming a constant proximal velocity from a drive motor variations in the distal velocity are typically related to storage of energy in the drive shaft in the form of torsion. If the catheter rotates at a velocity that is greater or less than the desired rotational velocity, reflected ultrasonic energy that is received from a particular location will be portrayed in the resulting image as being from an incorrect location.
With reference to FIG. 1, in the ideal, a catheter 10 is positioned coaxially with a vessel 12 for which ultrasonic intravascular images are to be formed. The catheter has a diameter that is relatively small compared to the diameter of the vessel. In this ideal situation, the catheter is rotated at a constant angular velocity, .omega..sub.0. Thus, there is a one-to-one correspondence between the anticipated directions of ultrasound transmission and reception and the actual directions of transmission and reception. Electrical signals generated in response to a reception of reflected energy may be accurately collected, processed and displayed.
Rather than a constant rotational velocity, FIG. 1 illustrates a "biphasic velocity profile," in which the catheter is rotating too quickly, .omega..sub.0 +.DELTA..omega..sub.1, for a portion of each revolution and too slowly, .omega..sub.0 -.DELTA..omega..sub.2, for another portion. By integrating the rotational velocity as a function of time, it is possible to obtain the angular position as a function of time. In FIG. 2, the anticipated rotational velocity is a constant, thereby producing a straight line 14 having a slope of .omega..sub.0. However, the actual biphasic velocity profile with an excessive rotational velocity over a first portion of each cycle and a diminished rotational velocity over a second portion is shown by plot 16. The difference between the anticipated rotational velocity 14 and the actual rotational velocity 16 is shown at plot 18, which is referred to as the angular error curve (AEC). The AEC determines the degree of error of an image.
The combination of nonuniform rotational velocity and eccentric catheter placement can lead to the distortion of the shape of a vessel wall. As an example, FIG. 3 shows an actual vessel wall 20 compared to the image of the actual vessel wall when a catheter has a biphasic velocity profile and is located eccentrically as illustrated. The dotted lines in FIG. 3 represent the ultrasound A lines, which are fired either too early or too late as a result of the incorrect velocity. For each ultrasound A line which is too early or too late, the angular position information to imaging equipment is incorrect. On the other hand, the range information to the imaging equipment is correct. Thus, the correct range information is rotated by the imaging equipment to the anticipated angular orientation in order to calculate the portrayal of the vessel wall range segment. The distorted image of the vessel wall is then constructed as the spline which connects all the "rotated" A lines. FIG. 3 illustrates an overestimation of the vessel area, as well as a distortion of the local curvature of the wall. Underestimations of the lumen area are also possible with different eccentric placements of the catheter.
Previous attempts to measure the position, or orientation, of the transducer have included a fluoroscope marker, as taught by Scribner et al. in U.S. Pat. No. 5,054,492, a magnetic resonance imaging-based tracking system, as taught by Dumoulin et al. in U.S. Pat. No. 5,271,400 and an external ultrasound-based system, as taught by Crowley in U.S. Pat. No. 5,131,397. While each of these systems operates sufficiently for its intended purpose, each system requires bulky and expensive additional equipment in order to perform the IVUS imaging of a vessel wall. Moreover, none of the systems recognizes the nonuniform rotational velocity problem or addresses a solution to the problem.
U.S. Pat. No. 5,243,988 to Sieben et al. describes the use of markers, preferably periodic variations in wall thickness of the catheter sheath, as a means for rotary encoding, but concerns exist. First, the numbers identified in the patent with regard to the sheath thickness and the transducer frequency are such that the walls are likely to be too thin to be resolved. Second, the abrupt changes in wall thickness may cause distortion of the ultrasound beam, due to refraction, unless the speed of sound in the sheath is close to that in water, which is not true for most plastics that are used to construct conventional sheaths. The distortion of the beam would result in degradation of image quality. Moreover, abrupt changes in wall thickness reduce the number of angular positions that can be encoded.
In a thesis entitled "Scanning Mechanisms for Intravascular Ultrasound Imaging: A Flexible Approach," Erasmus University, Rotterdam, 1993, by H. ten Hoff, non-uniformity of angular velocity was again addressed. Acoustic, capacitive, electromagnetic and optical techniques were considered as means for angle detection. Correction of the image was then performed in one of two ways. According to one method, an angle detection signal was used to measure or estimate the traversed angle between two successive directions of ultrasonic transmission, emitted at equal time intervals. The resulting information was then fed to display processing for correctly positioning the corresponding image-lines. H. ten Hoff concluded, however, that the tangential resolution became dependent upon the rotational angle, which diminished the image quality. The second method was one of using the angle detection signal to trigger the emission of ultrasonic pulses at equal traversed angle increments, so that image-lines were then generated periodically. The paper briefly referred to use of acoustic techniques to determine the angle detection signal, but focused upon optical determination because of a number of identified drawbacks to the acoustic solution. The identified drawbacks included low resolution of ultrasonic reflecting structures, multiple reflection, and shadowing.
What is needed is an ultrasound device and method by which the angular orientation of a catheter tip can be tracked in real time in order to reliably and repeatedly identify the position and properties of a specific anatomic structure, such as calcified plaque, and to adaptively correct angular velocity profiles which potentially extend over more than one rotational cycle.